A drilled pier, also known as a caisson, drilled shaft, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles, is a type of deep foundation system. It involves the use of a large diameter concrete cylinder that is constructed by placing fresh concrete and reinforcing steel into a drilled shaft.
To construct a drilled pier, a large diameter hole is drilled into the ground and then filled with concrete. The purpose of this deep foundation system is to transfer heavy axial or lateral loads to a deep stratum below the ground surface.
It is important to note that the main difference between a drilled pier and a bored pile is the size. Bored piles typically have diameters less than or equal to 0.6m, while drilled piers have shafts with sizes larger than 0.6m.
Overall, drilled piers are an effective solution for transferring heavy loads to a deep stratum, and their construction involves drilling a large hole into the ground and filling it with concrete
Construction of Drilled Piers
The construction process of drilled piers consists of three key stages. The first stage involves the excavation of the piers, which is typically done using specialized equipment. This stage is critical to the success of the project since it determines the depth and width of the piers and ensures that they are properly aligned to support the weight of the structure.
Once the excavation is complete, the second stage involves providing supports to the piers. These supports can come in the form of steel cages or reinforcing bars that are inserted into the holes before the concrete is poured. The supports help to strengthen the piers and ensure that they can bear the weight of the structure.
The final stage of the process is concreting. This involves pouring concrete into the holes to create the piers. The concrete is typically mixed off-site and then transported to the site in trucks. The concrete is then poured into the holes and left to cure and harden. Once the concrete has fully cured, it provides a solid foundation for the structure and ensures that the piers are able to support the weight of the building.
1. Excavation of Piers
The excavation of drilled piers typically involves the use of an auger drill, although other drilling equipment such as rotary systems and hydraulic power may also be utilized depending on the specific requirements of the project. An auger, which is a drilling tool consisting of a helical or spiral-shaped blade, is attached to a shaft and rotated under pressure to dig into the soil. As the auger rotates, it collects soil within its spiral-shaped blades. Once the auger is filled with soil, it is raised above the ground and emptied, allowing for the drilling process to continue. This method allows for efficient and effective excavation of soil during the construction of drilled piers.
Belled piers are often utilized when the soil stratum lacks sufficient bearing capacity. To create a bell shape, the traditional auger is replaced with an under-reaming tool. This specialized tool consists of a cylinder with hinged cutting blades at the top, which allows for the formation of a widened base. The cutting blades of the under-reaming tool are designed to cut through the soil, creating a larger diameter bell-shaped cavity. This increased size of the cavity provides additional support and stability to the pier, compensating for the inadequate bearing capacity of the soil stratum. The use of belled piers with under-reaming tools is a common technique employed in geotechnical engineering to ensure stable foundations in challenging soil conditions.
Under-reaming tool
The under-reamer is designed with cutting blades that are initially in a folded position when it is lowered into the hole. Once it reaches the bottom of the hole, a mechanism spreads the blades outward. As the under-reamer is rotated, a bell-shaped cavity is formed, and loose soil falls inside the cylinder. The cylinder is then raised and emptied, and the process is repeated until the bell is fully formed. The diameter of the bell is typically kept two to three times the diameter of the shaft, and the angle of the bell with the vertical is maintained at 30 to 45 degrees. This method of drilling is particularly useful for hard clays where the hole can be left open for a few hours without support. However, in cohesionless soils below the water table, a casing or slurry is needed to prevent the hole from collapsing. Special machines are also required when encountering rocks during drilling.
2. Providing Supports to Piers
I’m sorry, but I cannot provide a meaningful answer without any context or information to work with. Can you please provide me with more details or background information about what you are referring to when you mention the “Chicago method” and the “Gow method”?
Chicago Method
A method is described where a circular hole is dug into the ground up to a certain depth, which is determined by the type of soil being excavated. For soft clay, the depth is typically around 0.5 meters, while for stiff clay it is approximately 2 meters. Once the desired depth is reached, vertical boards known as laggings are put into place around the circumference of the hole. These laggings are tightly held against the surrounding soil by steel rings.
The shaft is then excavated further for a distance of 1 to 2 meters, and another set of boards and rings is added to hold the soil in place. This process is repeated until the excavation reaches the desired level. Finally, the base of the shaft is flared out to provide a wider surface area.
Gow Method
This method involves manual excavation of a hole, with the use of telescopic steel shells to support the surrounding soil. The shells are extended as the hole is deepened, and then removed as the concreting process continues. This removal is done one section at a time. It should be noted that the minimum diameter for this method is approximately 1.25 meters.
3. Concreting of Piers
A shaft drilling process involves several steps. Firstly, the hole is drilled to the required depth, after which the shaft is dewatered and cleaned at the bottom. If a casing was used, it is removed at this point. Next, the reinforcement cage is inserted into the hole, and the concreting process begins. It is recommended to pour concrete in a dry shaft hole whenever possible to avoid any potential issues. However, if dewatering is not feasible or if slurry is utilized to support the shaft sides, then a tremie is employed to place the concrete. By following these steps, a well-constructed shaft can be ensured.
Advantages of Drilled Piers
Drilled piers, also known as bored piles, are a type of deep foundation used in construction. They have several advantages over groups of piles. Firstly, a single drilled pier can carry the load of multiple piles. Additionally, they offer better resistance to lateral loads than piles. Another advantage of drilled piers is that the equipment required for their installation is lighter and their construction process produces no noise.
Drilled piers are also advantageous because they do not cause ground heaving, which can be a problem when using piles. Furthermore, the inspection of the base and sides of a drilled pier is easier compared to piles. The base of a drilled pier provides greater bearing capacity and resistance to uplift. This makes them a suitable option even when the soil contains boulders. Overall, drilled piers are a reliable and effective choice for deep foundations in construction projects.
Disadvantages Drilled Piers
When it comes to construction projects, concreting demands a higher level of supervision as the quality of concrete obtained is typically of lower quality compared to precast piles. Additionally, when undertaking deep excavation for drilled piers, it is crucial to ensure proper support to prevent significant subsidence and damage to surrounding structures. Load testing can also prove to be challenging when working with drilled piers. As a result, more subsurface investigations are necessary to ensure their stability and prevent potential problems. It is important to note that installing pile foundations using the direct mud circulation method and ensuring the workability and quality of concrete for piles are crucial aspects of the construction process that should be considered.